On Mars, a planet with no magnetosphere and an atmosphere roughly one percent as thick as ours, radiation is a real concern for any potential astronauts. Today there's good news today: Astronauts won't turn into radioactive goo the second they step on the surface, the Curiosity rover has confirmed. But, project scientists say, it'll be a little while longer before we really know what Mars' radiation means in human terms.

Since landing on the Martian surface just over 100 days ago, Curiosity has been collecting the first ever surface measurements of the Red Planet's radiation levels. Researchers announced their preliminary findings at a press conference today. "This has never been done on the surface of another planet before, much less Mars," said Don Hassler, principal investigator for the Radiation Assessment Detector (RAD).

Initially, Hassler said, researchers were surprised by the sensitivity of the RAD. They found that it was able to detect subtle, daily variations in radiation levels caused by changes in atmospheric pressure. "Basically we're finding the Mars atmosphere is acting as a shield for the radiation on the surface," Hassler said. "As the atmosphere gets thicker, that provides more of a shield, and we see a dip in our radiation dose." Between night and day, daily radiation levels on the surface can fluctuate between 3 and 5 percent, he says.

"We don't really experience this high-energy radiation environment on Earth that we do on Mars," say Hassler. "Mars is much more vulnerable and susceptible to this radiation."

So what does mean for future human exploration of the Red Planet? Despite the sensitivity of RAD. and the robust readings the science team is getting, the question of how long humans will be able to stay on the surface of Mars—and how insulated their space suits will need to be—is still up in the air. Currently, the radiation data taken from Curiosity is in arbitrary units, called the "radiation dose rate." That allows the scientists to compare how much the radiation on the surface fluctuates between Mars' night and day, but it doesn't allow them to compare it to known levels here on Earth. To do that, Hassler and his team need to closely examine the RAD measurements and calibration to translate the "radiation dose rate" into a a more usable metric, such as the commonly used rads.

In any case, Hassler assures us that it's not a question of "if" we'll learn about Mars' habitability, but rather a question of "how long?"

"Astronauts can absolutely live in this environment," he said. "The real question is if you add up the total contribution to the astronauts total dose when they do a Mars mission, [is that too much?]" Under current NASA regulations, all astronauts have a career limit of radiation exposure in the range of 1 to 4 sieverts. Taking into the account that astronauts will be exposed to radiation in the 18 months they'll be traveling to and from Mars, in addition to the six months they'll be spending on the surface, their radiation exposure will be an issue.